This application is related to commonly-assigned U.S. patent application, Ser. No. 12/969,861, filed Dec. 16, 2010; U.S. patent application Ser. No. 12/969,876, filed Dec. 16, 2010; and U.S. patent application Ser. No., 12/969,889, filed Dec. 16, 2010.
The present invention relates generally to turbomachines and more particularly to a method for enhancing the operational flexibility of a steam turbine during a startup phase.
Steam turbines are commonly used in power plants, heat generation systems, marine propulsion systems, and other heat and power applications. Steam turbines typically include at least one section that operates within a pre-determined pressure range. This may include: a high-pressure (HP) section; and a reheat or intermediate pressure (IP) section. The rotating elements housed within these sections are commonly mounted on an axial shaft. Generally, control valves and intercept valves control steam flow through the HP and the IP sections, respectively.
The normal operation of a steam turbine includes three distinct phases; which are startup, loading, and shutdown. The startup phase may be considered the operational phase beginning in which the rotating elements begin to roll until steam is flowing through all sections. Generally, the startup phase does not end at a specific load. The loading phase may be considered the operational phase in which the quantity of steam entering the sections is increased until the output of the steam turbine is approximately a desired load; such as, but not limiting of, the rated load. The shutdown phase may be considered the operational phase in which the steam turbine load is reduced, and steam flow into each section is gradually stopped and the rotor, upon which the rotating elements are mounted, is slowed to a turning gear speed.
For steam turbines equipped with cascade steam bypass systems, the startup phase begins with steam admission to the IP section using intercept valves. Subsequently, steam is admitted to the HP section. This process of admitting steam into the HP section, which completes the startup process, is generally referred to as forward flow transfer. Typically, steam flow through the HP and IP sections is balanced during forward flow transfer. The amount of steam flow is typically dependent on the operating Reheat (RH) pressure. This balanced flow transfer technique may, however, introduce a few issues.
This technique does not consider all of the physical parameters that may affect steam turbine operation during the startup phase. For example, during a hot start, a larger amount of HP steam flow is typically required to prevent high HP Section exhaust temperature. In order to balance the HP and IP steam flow after transfer, the IP steam flow prior to transfer should also be correspondingly higher. However, the increased IP steam flow prior to transfer may increase the axial thrust on the IP section, as insufficient steam may flow through the HP section to balance the higher IP steam flow. If, on the other hand, the IP steam flow prior to transfer is reduced to lessen the axial thrust load, the HP steam flow upon transfer will be lower than desired; and result in undesirably high HP section exhaust temperature. Rotor stress is another physical parameter that may be considered during forward flow transfer. In cold starts, very high HP steam flow may cause undesirable rotor stress. Therefore, multiple physical parameters that can affect the steam turbine should be considered before permitting steam flow into the IP and HP sections.
Some start-up strategies attempt to satisfy one or two of these physical parameter constraints. For example, but not limiting of, one strategy attempts to reduce rotor stresses introduced during the start-up process. This technique, however, does not address high HP section exhaust temperature caused by low steam flow into the HP section during forward flow transfer. Other starting strategies do not address all limiting physical parameters of existing steam flow balancing hot-start strategies, such as, but not limiting of, high HP section exhaust temperature.
These issues reduce the operational flexibility, require larger mechanical components, and potentially reduce the net-output delivered by the steam turbine during the startup phase. Therefore, there is a desire for a method for increasing the operational flexibility of the steam turbine during the startup phase.